(1) Field of the Invention
This invention relates to a flatbed-scanning type image reading apparatus and method for reading an image of an original color film by separating the light transmitted through the image into a plurality of color components.
(2) Description of the Prior Art
Among conventional color image scanning apparatuses are a drum-type apparatus and a flatbed-scanning type apparatus.
In the drum-type apparatus, an original color film which is provided on a peripheral surface of a drum and is rotated together with the drum, is photoelectrically picked up by a pickup head moving in an axial direction of the drum, and is separated into components of the three primary colors of blue (B), green (G) and red (R).
In the flatbed-scanning type apparatus, an original color film placed on a transparent table is photoelectrically read by a horizontally movable reading head which is equipped with a CCD line sensor, and is separated into components of the three primary colors.
Although a positive color film (reversal film) is generally used as an original film in the above color image reading apparatuses, a negative color film is also used mainly in the newspaper industry. This fact demands an image reading apparatus for reading both positive and negative color films. Further, reddish fogging, which is inevitable for a developed negative color film due to the color of the base thereof, should be prevented from adversely influencing color image reading accuracy.
In view of the above requirements, the conventional drum-type apparatus is equipped with circuitry for electrically compensating the color of the base of the negative film.
Since the drum-type apparatus employs a photomultiplier as a photoelectric converter in a pickup head, an enough dynamic range can be obtained with little possibility of getting noise. Accordingly, the use of the above circuitry has no adverse influence on the pickup accuracy.
In the case of the flatbed-scanning type apparatus, however, the use of the above circuitry hampers the reading accuracy for the following reason. This type of reading apparatus mainly employs a CCD line sensor. Since the CCD line sensor does not have an enough dynamic range in the reading head, the output level of the CCD line sensor is often substantially the same as the noise level when a highly dense image is read. Accordingly, amplifying the output may result in erroneously getting noise.
The image reading accuracy is especially low for the blue component when a halogen lamp is used as the light source, for the following reason. The intensity of the light transmitted through the negative color film is lower than that transmitted through the positive color film. The problem is that the difference between the light intensity through the negative film and that through the positive film is varied depending on the color component due to the reddish fogging: largest for blue and smallest for red.
The use of the above circuitry has another problem of complicating and enlarging the construction and increasing the manufacturing cost of the apparatus.